The Sony Auto-Reverse Walkman features before and after The Dude's magic carpet acid flashback caused by a swift punch to the head by one of Maude's goons.
The cassette tape, which appears to have been a customized recorded tape by The Dude, contains two clealy labeled sides:
Side A : Venice Beach League Playoffs 1987
Side B: Bob
Side A can be heard before the flashback, we hear bowling alley sound effects like the ball rolling followed by the pins falling down. Before each sound of the pins falling we see The Dude's wrist revolve mimicking the action of throwing a ball.
Side B, labeled simply Bob, refers to Bob Dylan and can be heard as soon as we enter the flashback with Bob Dylan's "The Man In Me". The song continues until we return to reality and The Dude is laying on the bare hard wood floor of his Venice Beach home.
Although it does not appear on screen, The Dude is wearing headphones and can be assumed to using the same Walkman. We can hear Elvis Costello's "My Mood Swings" is playing in his headset. In this scene he is visiting Maude's doctor whom she recommended for being thorough and with the promise that he would recieve no bill.